Lecture 18: Nervous System Organization

Question 1

Define the central nervous system

Answer 1

Definition:

• Unpaired, bilaterally symmetrical structures extending along the longitudinal axis of the midsagittal plane of the body.
• Structures arising directly from the neural tube.

Includes:
- Brain and Spinal cord

• See Slide 4

Question 2

Define the peripheral nervous system

Answer 2

• Definition:
• Made up of transmission pathways carrying information between the CNS and external/internal environments.
• Afferent (sensory) pathways: Carry information to the CNS.
• Efferent (motor) pathways: Carry information from the CNS.
• Includes:
• Cranial nerves (10-12 pairs)
• Spinal nerves (variable; 31 pairs in humans)

Question 3

Describe the autonomic nervous system (ANS)

Answer 3

• Definition:
• May be considered a subdivision of the PNS.
• Entirely motor.
• Innervates smooth muscle and glands (viscera)
• Includes two subdivisions:
• Sympathetic system (fight or flight): Also called thoracolumbar
• Parasympathetic system (feed and breed): Also called craniosacral
• See slide 7

Question 4

Describe the three parts of a neuron

Answer 4

1. Cell body:
   - That part of a neuron that encloses the nucleus and other organelles necessary to maintain and repair neuron.
   - Trophic unit
   - Perikaryon
   - Organelles: Nucleus, Golgi apparatus, RER, Ribosomes, which are called Nisl Substance…or something.
2. Dendrites: Receptive Unit
   - Branches off the cell body that carry information to the cell body.
   - Usually several to many.
   - Relatively short.
   - Often branched.
   - Have receptors for neurotransmitters.
   - Conduct local potentials.
3. Axon: Conductive Unit
   - That part of the neuron that carries information to another neuron or muscle cell.
   - Usually relatively long
   - Single.
   - Conducts action potential
   - Ends in short branched processes called telodendria
   - May have collateral branches
   - Cell membrane = Axolemma
   - Cytoplasm = Axoplasm
   - Contains: Mitochondria, Neurofilaments, Neurotubules
   - Capable of axonal transport: Anterograde, Retrograde, Slow transport, and Fast transport
   - Covered by neurolemma: Made up of Schwann cells.
   - Often myelinated, Myelin is formed by Schwann cells.
   - Note: axon is only part of neuron that is ever myelinated.

• See Slides 10-11

Question 5

Define the following terms:

Nerve, Tract, Commissure, Nucleus, Ganglion, White Matter, and Gray Matter

Answer 5

• Nerve: Bundle of fibers in the PNS
• Tract: Bundle of fibers in the CNS
• Commissure: Tract in the CNS that crosses from one side to the other
• Nucleus: Aggregation of dendrites and nerve cell bodies in the CNS
• Ganglion: Aggregation of dendrites and nerve cell bodies in the PNS
• White matter: Areas of myelinated axons.
• Gray matter: Areas of unmyelinated axons, cell bodies, and dendrites.

Question 6

Describe the nerve pathways, Synapses

Answer 6

• Components:
• Presynaptic membrane: With synaptic vesicles filled with neurotransmitters.
• Synaptic cleft:
• Postsynaptic membrane: With receptors for neurotransmitters.
• Monosynaptic pathways.
• Polysynaptic Pathway

Question 7

Describe the nerve pathway, the reflex arc.

Answer 7

• Afferent Sensory Pathway
• Somatic
• Visceral (splanchnic)
• Efferent (motor) pathways:
• Somatic
• Visceral (splanchnic)
• Association neurons (interneurons)

Question 8

Describe neuroglial cells

Answer 8

• Schwann cells (from neural crest cells):
• Myelinate axons in the PNS.
• Astrocytes (from neural crest cells):
• Function to physically support neurons.
• Channel materials between capillaries and neurons. (Blood-brain barrier.)
• May serve to guide neurons during embryonic building of cerebral cortex.
• Microglial cells (from monocytes):
• Transform into phagocytes within CNS
• Oligodendrocytes (from neural crest cells):
• Function to myelinate axons within CNS
• Ependymal cells (from neural crest cells):
• Line ventricles of brain

Question 9

Describe neural tube development

Answer 9

• Stages in neural tube development:
• Neural plate
• Neural folds
• Neural tube
• Subdivision of cranial end of tube:
• Tripartite brain
• Pentapartite brain
• See Slides 18-22

Question 10

What are the three divisions of a tripartite brain

Answer 10

• Prosencephalon
• Mesencephalon
• Rhombencephalon
• See Slide 23

Question 11

Describe the three subdivisions of the pentapartite brain

Answer 11

• Prosencephalon:
• Telencephalon (most anterior)
• Diencephalon
• Mesencephalon
• Rhombencephalon:
• Metencephalon
• Myelencephalon
• See Slide 24- 25

Question 12

What are the adult derivitives of the subdivisions of the pentapartite brain?

Answer 12

1. Telencephalon primordia:
   - Lumina: Lateral ventricles (I, II)
   - Floor: Basal ganglia (nuclei) Olfactory lobes and nerves
   - Roof: Cerebral hemispheres
2. Diencephalon primordia:
   - Lumen: Third ventricle
   - Roof: Epithalamus
   - Walls: Thalamus
   - Floor: Hypothalamus and infundibulum
3. Mesencephalon primordia:
   - Lumen: Cerebral aqueduct (of Sylvius)
   - Roof: Tectum: Corpora bigemina in lower vertebrates, Corpora quadrigemina in higher vertebrates:
   - -Superior and inferior colliculi
   - Floor: Tegmentum
4. Metencephalon primordia:
   - Lumen: Part of fourth ventricle
   - Roof: Cerebellum
   - Floor: Pons
5. Myencephalon primordia:
   - Lumen: Rest of fourth ventricle
   - Main part: Medulla oblongata
   - Roof: Posterior choroid plexus

• See Slides 30-32

Question 13

Describe the histogenesis of the neural tube

Answer 13

• Initial tube wall
• = Pseudostratified epithelium:
• • Single layer of cells, but cells are of different heights.
• • All cells are in contact with a basement membrane.
• Outermost membrane =
• External limiting membrane
• Some neuroepithelial cells remain attached to the basement membrane and will form a single layer of ependymal cells that will line the entire ventricular system and the neural canal.
• Other cells lose contact with the basement membrane and will migrate past the ependymal cells to form a new outer layer of densely packed cells collectively called the:
• Mantle layer:
• Cells that make up the mantle layer are: NEUROBLASTS
• Note that mantle layer is still covered by the external limiting membrane.
• Neuroblasts in the mantle layer will begin to grow processes (axons) that will form a new outer layer:
• Marginal layer
• The marginal layer is also located beneath the external limiting membrane.
• The marginal layer will form the white matter (myelinated axons) of the spinal cord and the brain.
• The mantle layer forms the gray matter of the brain and spinal cord (except for the cortices).
• See slide 35-36